Aging is associated with an increased susceptibility to disease. It is likely that age-associated changes in the immune systems, in the mechanisms that regulate cell growth and proliferation, and in the response of arterial cells to injury and diet, contribute to the increased prevalence of infection, cancer and cardiovascular disease in the elderly. The major focus of this multidisciplinary study of the immunobiology of aging is the effect of age on the structure and function of the immune system. The effect of age on: the differentiation of lymphocytes; the cell-to-cell interactions in the immune response; the molecular basis of lymphocyte proliferation; and the response of humans to influenza vaccination will be studied in man and experimental animals. The age-associated changes in arterial cells that may play a role in the pathogenesis of atherosclerosis will be explored. Finally, the effect of age on chromosomal structure will be studied in order to gain further understanding of the molecular basis of the age-associated changes in cell function.